One of the harder problems I have worked on: a site lost its Google clicks on a single day, then was slowly deindexed almost entirely apart from a handful of pages. The commercial pages, the ones that make the money, were gone.
What we ruled out
The obvious culprits were checked and came back clean. No AI-generated content. No stray noindex tag. Nothing wrong in robots.txt. No x-robots-tag header quietly excluding anything. Then you open Search Console and the property looks healthy, which is the part that makes this kind of failure so disorientating.
That is worth sitting with. Search Console reporting healthy is not the same as indexed. It is the same blind spot behind the indexing problem most ecommerce SEO ignores, where teams publish page after page that Google never adds to the index and nobody checks.
Where to look when the obvious answers are clean
When the standard checks pass, work down the layers instead of going in circles. Compare the URLs Google actually has against your sitemap rather than trusting summary counts. Read the crawl stats around the drop date and establish whether Google stopped fetching or started fetching and rejecting. Check what your server returns to Googlebot specifically, because a bot-only error or an aggressive firewall rule will never reproduce in your own browser.
Then look at infrastructure, because crawl budget follows server response, which is exactly why time to first byte matters more than image compression. Confirm redirects and canonicals are consistent too, the unglamorous items on the technical checklist most agencies never get implemented.
Ask for help early, and ask publicly
I put this problem out in the open and asked for the best people to look at it, including a full walkthrough of crawl stats and URL comparisons. That is not weakness. You do not need to be the smartest person in the room, you need the right people around you, and fresh eyes find what your own team has stopped seeing.
It is also the difference between an agency that reports and one that diagnoses, which is why the strategy deck matters less than what actually got delivered and why the partner matters more than the platform.
If your traffic dropped off a cliff and nobody can tell you why, book a call.